1. Field of the Invention
The present disclosure generally relates to imaging and, more particularly, to an image forming apparatus having a function of measuring colors of a measurement image.
2. Description of the Related Art
Image quality of the image forming apparatus includes granularity, in-plane uniformity, text quality, and color reproducibility (including color stability). In recent years in which a multi-color image forming apparatus has become widely used, the color reproducibility in some cases is the most significant image quality.
Humans have a memory of expected colors (in particular, of a human skin, blue sky, and metal) based on experience, and if a color exceeds an allowable range of such an expected color, a viewer develops a feeling of strangeness. Such colors are referred to as memory colors, and reproducibility of the memory color has become a concern when outputting a photograph.
Further, an increase in the demand for the color reproducibility (including image stability) with respect to the image forming apparatus is not limited to that of the photographic image. In the case of a document image, there is an increasing demand from office users who feel discomfort in a color difference between the output from the image forming apparatus and that on a monitor, and from graphic artists who pursue color reproducibility of a computer graphics (CG) image.
To satisfy such user demands for the color reproducibility, Japanese Patent Application Laid-Open No. 2004-086013 discusses an image forming apparatus which reads, using a measurement unit (i.e., a color sensor) disposed in a conveyance path of a sheet, the measurement image (i.e., a patch image) formed on the sheet. Such an image forming apparatus feeds back into process conditions such as an exposure amount and a developing bias, a result obtained by the color sensor reading the patch image. Constant density, gradation, and tint can thus be reproduced.
However, detection accuracy of a color value by the color sensor discussed in Japanese Patent Application Laid-Open No. 2004-086013 becomes degraded by an output fluctuation of a light source due to a change in environmental temperature. To solve such a problem, calibration may be performed by arranging a white reference plate in a position opposite to the color sensor, so that the color sensor measures the white reference plate, and corrects a detection value of the color sensor.
More specifically, spectral reflectivity R (λ) of the patch image may be obtained as R (λ)=P (λ)/W (λ), wherein W (λ) is reflected light from the white reference plate, and P (λ) is the reflected light from the patch image.
If the spectral reflectivity of the patch image is obtained using the white reference plate, a problem may occur in which there is discoloring of the white reference plate due to irradiation of light so that an error is generated in the measurement value. When the white reference plate contains a material which is discolored by an oxidation effect of light, the discoloring occurs in the white reference plate irradiated with light.
Since the white reference plate is irradiated with light while performing calibration, the discoloring gradually progresses during irradiation of the light in each calibration. As a result, the error in the measurement value gradually increases.